The effects of stress relieving heat treatment on the microstructure and residual stress of Inconel 718 fabricated by laser metal powder bed fusion additive manufacturing process

In this study, the microstructure and residual stress of the Inconel 718 parts, which were fabricated by laser metal powder bed fusion additive manufacturing process, in as-fabricated and stress-relieved conditions were investigated by metallographic analysis and microhardness test. Fine equiaxed and columnar cellular dendrites were revealed in the Transverse plane, Frontal plane, and Sagittal plane, respectively. Laves phases were found in the interdendritic regions and interlayers. After the stress relief heat treatment, the Laves phases has partly solved into the γ matrix and the microstructure became relatively homogeneous, which leads to the significant increase (~19%) in the microhardness. Residual stress was unevenly distributed in the parts, and no notable difference was found between the Frontal plane and Sagittal plane. The maximum absolute compressive residual stress dropped from 378.4 MPa to 321 MPa after stress relief due to the homogenization of the microstructure